The present invention generally relates to crucible fabrication. Specifically, the present invention relates to methods and apparatus for making quartz glass crucibles that are suitable for use in pulling silicon single crystals for semiconductor applications.
There are several methods for making single crystal silicon materials. One such methodxe2x80x94the xe2x80x9cCzochralskixe2x80x9d (CZ) methodxe2x80x94has been widely adopted for producing single crystal silicon materials for semiconductor applications. In this method, polycrystalline silicon is melted in a vessel and a single-crystal seed crystal is dipped at its tip end portion into the molten silicon. The seed crystal is then xe2x80x9cpulledxe2x80x9d while being rotated so that a single crystal from the melt is grown on the seed crystal with the same crystallographic orientation.
A crucible is one vessel commonly employed in this pulling operation for making ingots of silicon single crystals. Crucibles are typically configured in a bowl shape, with walls and a bottom, to contain the molten silicon during the pulling operation. Crucibles may be made of any material which adequately contains the molten silicon without contaminating it, yet introduces a desired amount of oxygen into the molten silicon. One of the most widely used materials for such crucibles is silica or quartz glass.
These walls of the crucibles normally have double layer structures, with an inner layer and an outer layer. Normally, the outer layers of the crucibles are translucentxe2x80x94for increased scattering of the radiation from heater elementsxe2x80x94and therefore contain numerous tiny bubbles. When the bubbles open during the crystal-pulling operation, however, they can damage the silicon ingots because any generated particle can travel from the bubble to melt-ingot interface and damage the crystal structure of the pulled ingot. To reduce this impact of bubbles in the outer layer, as well as for other reasons, the bubble-free inner layer is often formed between the outer layer and the melt.
The inner layer of the crucible wall should also as pure as possible to minimize the possibility of contamination of the ingot during the crystal-pulling operation. During the crystal-pulling process, portions of the inner layer can dissolve into the silicon melt. Silicon and oxygen, which are the main components of the inner layer, are not harmful to the silicon melt. Indeed, the dissolved oxygenxe2x80x94a main source of oxygen in the wafers cut from the ingot-acts as a mechanical strengthening element and as a gettering agent in the wafers. Other components such as impurities, however, are also transferred to the melt during the crystal-pulling operation.
One of the numerous methods known for preparing crucibles with inner layers which are substantially bubble-free is described in U.S. Pat. Nos. 4,416,680, 4,956,208, and 4,935,046, the disclosures of which are incorporated herein by reference. Other methods include those described in Japanese Patents JP1-197381 and JP1-197382 (which are modifications of the method described in U.S. Pat. No. 4,416,680), the disclosures of which are incorporated herein by reference. There are also numerous methods known for purifying the quartz grain used to make crucibles as well as electrolyzing finished crucibles, thereby making crucible walls with a xe2x80x9cpurexe2x80x9d inner layer. See, for example, respectively U.S. Pat. Nos. 5,637,284 and 4,874,417, the disclosures of which are incorporated herein by reference.
Despite the above efforts, currently available crucibles still have many problems. One important problem, as mentioned above, is the purity. Another important problem is that after running a long period, such as more than 100 hours, the inner surface of the crucible becomes rough. It is believed that the inner surface becomes rough either through merging of cristobalite rings formed on the inner surface during crystal pulling or through spot devitrification. When the inner surface becomes rough, the crystal structure of the ingot can suffer.
The semiconductor industry is moving towards larger wafer sizes. Consequently, the diameter of the ingots from which the wafers are cut must also be larger. But both of the problems mentioned immediately above become more pronounced as the size of the ingots becomes larger. To obtain larger diameter ingots, the CZ process must be run longer, increasing the possibility of the inner surface become rougher and more impurities dissolving into the melt from the inner layer of the crucible wall.
The present invention provides silica or quartz glass crucibles containing inner layers that are substantially pure and substantially bubble-free. When used in a CZ-crystal growing process, the inner layers of such crucibles are substantially stable against roughening and spot devitrification. The present invention also provides methods for making crucibles with these characteristics.
The inner layer is formed by a method which first forms a web structure using quick and high fusion power at the initial stage of fusing quartz grain to a silica glass crucible. The web structure is then converted to a continuous layer, thereby minimizing or eliminating bubble formation, bubble growth, surface roughening, and spot devitrification. The inner layer is also formed using a gettering agent which getters alkaline and alkaline-earth elements while the inner layer is formed. The alkaline and alkaline-earth elements are gettered on an innermost surface of the inner layer which is later removed, leaving an inner layer with relatively few impurities.